1. Field Of The Invention
The present invention relates to Cathode Ray Tube Video displays, and more particularly to brightness control systems for high performance Cathode Ray Tube displays.
2. Description Of The Prior Art
The normal mode of operation of Cathode Ray Tube (CRT) Video displays involves applying a video signal, which is a linear function of the original scene brightness, to either the CRT grid or cathode. In the case of most computer generated displays, the video signal is applied in equal voltage steps to represent progressive changes in scene brightness. Because of the nonlinear transfer function of the CRT grid-cathode voltage to screen brightness characteristic and the great adaptability of the human eye, this type of control is suitable for many applications. The interaction of separate brightness and contrast controls also is tolerable in most situations. However, in critical high quality display applications with rapidly changing ambient light conditions, the above characteristics are unacceptable.
The necessary characteristics for high performance aircraft cockpit displays include automatic brightness control as a function of ambient level, minimum interaction between brightness and contrast controls, and a gamma characteristic which provides a reasonable approximation to the variation of brightness difference threshold of the eye. The gamma characteristic is the slope of the brightness/input voltage (exponential) transfer characteristic of the CRT. This latter requirement derives from the necessity of rapidly recognizing the maximum number of shades of gray in the video display, under widely changing conditions of ambient light level.
It is an object of the present invention to provide automatic brightness control for a CRT video display as a function of ambient light level. It is another object to provide automatic brightness control as a function of ambient light level for a high performance video display. It is another object to provide automatic brightness control for a CRT video display with minimum interaction between brightness and contrast control. It is a further object to provide an automatic brightness control system for a high quality CRT display which produces a gamma characteristic providing a reasonable approximation to the variation of brightness difference threshold of the eye.